As has been said...neither is better than the other. They're just different. Both breeds have very different temperaments and needs.
Boxers, for example, needs a lot of exercise, whereas Rottweilers (although active) don't need anywhere near as much.

You need to be very specific when asking yourself what you want in a dog.
How much exercise (and what type of exercise) are you willing to give it?
How much grooming (that doesn't just mean brushing the coat...that includes nail clipping, teeth brushing, wrinkle cleaning etc...)?
How much socialisation and training are you willing to give it? If you get a more protective breed you need to give it a lot more training and socialisation in order to make it a good member of society.
Where will the dog live? Will it be inside most of the time, with access to a yard? Will it be an entirely indoor dog (perhaps in an apartment)? Will it live outside? Will the yard be fenced? Will you have a dog run for it instead?
How much money on vet care are you willing to devote? (some breeds are more prone to health problems than others...Boxers, for example, have a high incidence of cancer in the breed).

Write down a list of all the things you expect of your perfect dog and then try to find one that matches that list. You can't mould a dog to fit you, you have to find one that already fit in the first place.

Good luck on your search though. You may find a different breed suits you better...you never know. At the very least you'll be able to figure out if a Boxer or Rottweiler better suits you.

Cass.

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I've been frosted!Say NO to animals in pet shops!Rescue a friend instead.
"Raise them to be gentle and they will be gentle. Raise them to be vicious, and they will be vicious.
Problems lie not in the breed, but in the people." - Dr. Carl Semenic

That totally depends on what you are looking for in a dog. Like already asked;
How much exercise will your provide for a dog?
Do you already have a dog?
Is this your first dog?
What other pets do you have in your house?
How experienced are you with training?
How do you feel about shedding and grooming?
How long are you gone during the day?
Do you have children?
Are you a fairly active person?

Can you elaborate in why you are looking at these breeds and what you expect in a dog?